1. Field of the Invention
This invention relates generally to a device for cleaning an image carrier for an image forming apparatus such as a printer or a copying machine in which electrophotographic technology is adopted, and more particularly to a cleaning device of the type mentioned which employs a conductive cleaning brush therein.
2. Description of the Prior Art
In recent years, page printers represented by laser printers have been become major recording apparatus from the viewpoint of their quietness, high recording speed, and high quality printed results. Most of such page printers form a picture image using electrophotographic technology and generally have a construction as shown in FIG. 2. Referring to FIG. 2, a recording device shown includes a photosensitive member 1 serving as an image carrier located substantially at the center within a housing, with a charger 2, an optical writing device 3, a developing device 4, a transfer device 5, a fixing device 6, a charge removing device 7 and a cleaning device 8 are arranged in this order around the photosensitive member 1. A supply paper cassette 9 and a discharged paper tray 10 are removably mounted on the housing of the recording device.
In a process of forming a picture image with such a recording device as described just above. The photosensitive member 1 which is rotating in the direction indicated by an arrow mark A in FIG. 2 is charged with electricity by the charger 2. An electrostatic latent image corresponding to information of a picture image to be recorded is formed on the photosensitive member 1 by the optical writing device 3. Colored fine particles of an average particle size of 10 to 20 microns (hereinafter referred to as "toner powder") are applied by the developing device 4 so as to selectively adhere to the photosensitive member 1 in accordance with a pattern of the electrostatic latent image in order to convert the image into a visible image. A record sheet 11 of paper is supplied from the supply paper cassette 9 and transported along a substantially horizontal paper transport path extending from the supply paper cassette 9 to the discharged paper tray 10. The visible image of toner powder on the photosensitive member 1 is thus transferred by electrostatic attractive force of the transfer device 5 to a lower face of the record sheet 11 being transported between the transfer device 5 and the photosensitive member 1. The visible image of toner powder on the record sheet 11 is fixed by heat or pressure. The photosensitive member 1 having passed the transfer area rotates past the charge removing device 7 by which the remaining charge on a surface thereof is removed using light or discharge. Toner powder still remaining on the photosensitive member 1 is removed by the cleaning device 8, thereby completing the picture image forming process for one page.
Prior art cleaning systems, as illustrated in FIGS. 3, 4 and 5, have been conventionally put into practical use as methods of removing remaining toner powder. FIG. 3 illustrates a system called a blade cleaning system wherein a blade 15, made of an elastic material such as polyurethane rubber, is pressed at a predetermined angle against a surface of a photosensitive member 1 to mechanically scrape toner powder off the photosensitive member 1.
FIG. 4 shows another system called blower cleaning system wherein a brush roll 13' and an absorbing fan 17 for collecting toner powder are employed. The brush roll 13' is formed by implanting a large number of insulating yarn strings made of a high-molecular polymer such as, for example, nylon or acrylic resin into a ground fabric. The opposite side of the ground fabric is coated with a bonding agent by means of such a back coating device 18 as shown in FIG. 7. Then the ground fabric is wrapped around a roll member. The brush roll 13' is disposed at a position at which the brush fibers contact with a photosensitive member 1 which is rotating in the direction indicated by an arrow mark A in FIG. 4. As the brush roll 13' is rotated in the direction, for example, indicated by an arrow mark B in FIG. 4, the brush thereof is slidably contacted with the photosensitive member 1 whereupon it is contacted with toner powder T remaining on the photosensitive member 1. During the mechanical scraping of the toner powder T off the photosensitive member 1, an electric charge is imported to the surface of the brush fibers of the brush roll 13' due to frictional contact with the photosensitive member 1 during high speed rotation of the brush roll 13' to electrostatically remove the toner powder T from the photosensitive member 1. The toner powder transferred to the brush 13' is then transported in the direction indicated by an arrow mark D in FIG. 4 by the absorbing fan 17 and collected into a collection toner box not shown, thereby enabling continuous cleaning of the photosensitive member.
FIG. 5 shows an improvement to the blower cleaning system described above which is called insulating brush cleaning system. This system makes use of an insulating brush 13' to remove toner powder from a photosensitive member in a similar manner as in the blower cleaning system, but differs from the blower cleaning system illustrated in FIG. 4, in the method of collecting the toner powder attached to the brush 13'. In particular, according to the method, toner powder attached to the brush 13' is collected, for example, by means of a metal roll 14 which is contacted with the brush 13' to cause toner powder attached to the brush 13' to be transferred to the metal roll 14. More particularly, the method of transferring toner powder to the metal roll might incorporate a bias voltage of a polarity opposite to the polarity of charge of toner powder applied to the metal roll to cause toner powder to be collected by an electrostatic attracting force. Alternatively, a magnetized toner powder and a magnetic roll of opposite polarities might be used to cause toner powder to be magnetically attached and collected. These methods are improved so as to prevent an increase in size of an apparatus which is a defect of the blower cleaning system.
Finally, toner powder attached to the metal roll described above is mechanically scraped off the metal roll 14 and collected by means of a scraper 16 which is disposed in normal contact with the metal roll 14. In this way, member can be continuously cleaned by the metal roll.
However the conventional cleaning systems described above, have significant problems as described below. First, according to the blade cleaning system, a blade must be pressed against a photosensitive member under a high pressure in order to attain a high cleaning performance. The blade cleaning system has such a structure that the blade is pressed at an acute angle against the photosensitive member to clean the photosensitive member only with a mechanical scraping off force. If the cleaning operation is repeated in such a manner as described just above, not only is toner powder removed from the photosensitive member but the surface of the photosensitive member itself is gradually abraded also as it is slidably contacted by the blade. Accordingly, damage to the photosensitive member is so significant that a long life of the photosensitive member cannot be assured. Second systems which employ an insulating brush as in a blower cleaning system or an insulating brush cleaning system, do not damage the photosensitive member as significantly as that of the blade cleaning system. The insulating brush is rotated at a high speed to provide an electric charge to the brush through frictional contact thereof with the photosensitive member, toner powder, to electrostatically remove the toner powder. Nevertheless, some damage to the photosensitive member cannot be avoided. Improvement in life of the photosensitive member cannot be anticipated. Third both systems require a mechanism is required for rotating the brush at a high speed which increases, the number of parts needed and, the production cost. In addition, cumbersome problems of production of noises and countermeasures for the same are also involved. Finally that toner powder attached to the brush is dispersed by a centrifugal force which contaminates the inside of the image forming apparatus.
FIG. 6 shows a cleaning system which employs a conductive brush and resolves the problems described above. The cleaning system is constructed such that a positive DC bias voltage is applied to a cleaning brush formed from conductive fiber to remove toner powder of a negative charge property remaining on a photosensitive member after transfer of a visible toner powder image. Since remaining toner powder is removed by applying a bias voltage to the conductive brush, it is possible to lower the rotational speed of the brush. Thus cleaning system of FIG. 6 can resolve the various problems described above with said insulating brush cleaning systems, Including the improvement in life of the photosensitive member.
However, the cleaning system, wherein a bias voltage is applied to a conductive cleaning brush, in described in FIG. 6 above still presents the following problems when it is put into practical use. After development of the image, the negatively charged toner powder is transported to a transfer area while maintaining its negative charge, and in the transfer area, it is electrostatically adhered to record paper which has been charged to a positive potential by positive corona discharge. Toner powder which has not been transferred to record paper 11 inevitably remains on the photosensitive member 1, and part of this toner powder is reversely charged to the positive polarity by an influence of the positive corona used in transfer. The distributions are of particle charge illustratively shown in FIG. 8a and FIG. 8b in which the polarities of charge of toner powder and the distributions of quantities are illustrated by way of graphs. Referring to FIG. 8a and FIG. 8b, the polarity of part of toner powder having the negative polarity after development 1 is FIG. 8a reversed to the positive polarity after transfer see FIG. 8b, and as a result, toner powder which is charged to the positive polarity and toner powder which remains charged to the negative polarity will be present in a mixture on the photosensitive member. Accordingly, even if the photosensitive member is cleaned by a conductive cleaning brush to which a positive voltage is applied, the toner powder charged to the positive polarity will not be removed from the photosensitive member 1 and therefore, complete cleaning of the photosensitive member will not be attained.
In order to attain effective cleaning, it is necessary to set conditions such that toner powder may not be charged to the positive polarity upon transfer. However setting the conditions, is very difficult because the transfer makes use of a very unstable phenomenon of discharge. Moreover, positive charging of the toner powder occurs automatically as toner powder occurs as transfer corona leaks through the record paper. Furthermore transfer corona leakage appears more intensely as the record paper absorbs moisture to decrease the electric resistance thereof. Consequently the number of particles of toner powder which are charged to the positive polarity increases progressively as the humidity increases environment. As a result, as the environmental conditions vary, the toner charge polarity will vary, and the optimum bias voltage to be applied to the conductive cleaning brush will require adjustment to attain effective cleaning. A mechanism would be required which continuously measures environmental conditions and applies a feed-back to the set conditions. This requirement would complicate the system and increase the production cost.